1. Field of the Invention
The present invention relates to a hydraulic control apparatus of an automatic transmission to be mounted, for example, on a vehicle and, more specifically, to a hydraulic control apparatus of an automatic transmission for establishing transmission gear speeds by engaging a predetermined number of frictional engagement elements and preventing simultaneous engagement of a predetermined number or more of frictional engagement elements.
2. Description of the Related Art
In general, in a field of a multispeed automatic transmission to be mounted on a vehicle or the like, those configured to establish transmission gear speeds by engagement between, for example, two clutches and brakes (frictional engagement elements), and change the speed by changing a grip of the clutches and brakes go mainstream. In addition, in recent years, in the field of the multispeed automatic transmission, those improved in performance of a linear solenoid valve, and employing a configuration in which a hydraulic pressure regulated by the linear solenoid valve is supplied directly to hydraulic servos of the clutches and the brakes as an engagement pressure go mainstream.
In the multispeed automatic transmission as described above, for example, when three or more clutches and brakes are engaged simultaneously, a contradiction arises in a rotating state of a change gear mechanism, which may disadvantageously result in so-called a stall state. However, as described above, since the engagement pressure is controlled by the plurality of linear solenoid valves individually, if something unusual (for example, disconnection, short circuit, valve stick) occurs in the linear solenoid valves, unintended clutch or brake may engage, that is, a state in which three or more clutches and brakes are going to engage simultaneously may result.
Accordingly, fail-safe valves (600, 700) configured to allow inputs of engagement pressures from three linear solenoid valves and allow passage of the engagement pressure of one of the linear solenoid valves therethrough are provided and, when three of more engagement pressures are applied simultaneously, one of the engagement pressures passing therethrough is blocked to prevent the three or more clutches and brakes from engaging simultaneously is proposed (see JP-A-63-210443 (Patent Document 1)).
However, since the above-described fail-safe valves are configured to block the one of the three engagement pressures when the three engagement pressures are input, a number of the fail-safe valves are required for preventing the simultaneous engagement in the respective transmission gear speeds in automatic transmissions having multiplied speeds like, for example, those with forward eight speeds. In addition, formation of a number of oil channels is required for allowing inputs of the three engagement pressures to a number of the fail-safe valves. Consequently, there arises a problem of deterioration in terms of vehicle-mountability as a result of upsizing of the hydraulic control apparatus.
Furthermore, when causing the engagement pressure to pass through the fail-safe valve for allowing the engagement pressure to be blocked, the fail-safe valve is mandatory interposed in a range from the linear solenoid valve to hydraulic servos of the clutches and brakes, and hence resistance in a duct line during a supply of the engagement pressure under the normal state is increased, that is, the hydraulic responsiveness is lowered, and hence controllability of the clutches and the brakes is deteriorated, so that there arises a problem such that deterioration of transmission response or increase in generation of engagement shock may occur.